The odds of a fake wheat penny are slim to none. Nobody counterfeits regular circulation issue cent coins.
If you have celiac disease and you eat wheat, you'll get sick. That is how you can tell that it is made of real wheat.
Those would surely have been searched at some point I would think. Probably a better chance of snagging something in the way of varieties and the like. Granted finding wheats is getting slimmer and slimmer but I personally wouldn't buy any knowing that they're likely already searched. Good to know though for when I dump my hoard one day (and mine have been searched already of course).